1. Field of the Invention
This invention relates to a polymer water injection foaming devolatilizing method, and an apparatus for practicing the method, and more particularly to a novel improvement to remove volatile components from a molten polymer material with high efficiency.
2. Related Art
Heretofore, as for a conventional water injection foaming devolatilizing method, and a conventional apparatus for practicing the method, an extruding machine designed as shown in FIGS. 2 through 5 is employed. In FIG. 2, reference numeral 1 designates a cylinder showing a part of the upper portion of a twin screw type extruding machine 2. In the upper surface of the cylinder 1, a material supply port 3, a water injection port 4, and a vent port in the stated order as viewed from upstream. And in the cylinder 1, two screws which turn in the same direction are rotatably provided in such a manner that they are engaged with each other.
The cylinder 1 has a filling zone 10, a water injection zone 11 having a water injection port 4, and a devolatilizing zone 12 having the vent port 5 in the stated order as viewed in the direction of flow. The aforementioned screw 6 in the filling zone is made up of a full-flight screw 6a, the screw in the water injection dispersion zone 11 is made up of a second ring 13, a plurality of kneading-dispersing screws 14, and a first ring 15 which are arranged in the stated order as viewed in the direction of flow, and the screw 6 in the devolatilizing zone 12 is made up of a full-flight screw 6b.
FIG. 3 shows a pressure-reduction expansion zone 17. More specifically, in the structure shown in FIG. 3, a pressure-reduction expansion zone special ring 16 (shown in FIG. 4) is provided immediately behind the first ring 15 of the devolatilizing zone.
Under the condition, the polymer material supplied from the material supply port 3 is being extruded by the screw 6 while being molten and kneaded, the water injected from the water injection port 4 is dispersed in the high temperature molten polymer material under high pressure which is filled in the injection water dispersion zone. This high pressure is maintained by the extruding action of the screw 6 located upstream of the second ring 13 and by the damming action of the first ring 15, and it is higher than the saturated steam pressure of water when the injected water is dispersed in the high temperature molten polymer material. The devolatilizing zone 12 located downstream of the first ring 15 is in vacuum state. Hence, when passing through the first ring 15, the pressure of the molten polymer material is abruptly reduced, and the water dispersed in the molten polymer material is abruptly foamed. In this case, the volatile components contained in the molten polymer material start to disperse into the foams through the foam/polymer interface from the instant the foaming phenomenon occurs. On the other hand, in the foamed molten polymer material, the foams therein are broken by the shearing action of the screw 6 in the devolatilizing zone located downstream of the first ring 15. As a result, the volatile components in the foams are dispersed outside of the molten polymer material, and discharged outside through the vent port 5.
On the other hand, by means of the pressure-reduction expansion zone special ring 16 located immediately behind the first ring 15, the molten polymer material is slowly reduced in pressure after passing through the first ring 15, so that the foams grow sufficiently; that is, they are long in retention time. As a result, the molten polymer material is sufficiently degassed.
The conventional water injection foaming devolatilizing method and apparatus are designed as described above. Therefore, they suffer from the following problems. First, since the extruding machine employs a pair of rings, and one of the rings is provided upstream of the water injection dispersion zone, the resistance of flow thereat is high. Hence, when the molten polymer material passes through the clearance between the ring and the cylinder, the pressure of the molten polymer material is reduced, so that the water injection pressure and the amount of extrusion are not high enough. Second, at the clearance section of the ring, shearing heat is generated. Hence, it is impossible to improve the devolatilizing efficiency; that is, it is impossible to obtain a large number of revolutions of the screw, a high water injection pressure, and a high water dispersion effect. Third, the region between the pressure-reduction expansion zone special ring 16 located downstream of the pressure-reduction expansion zone and the first ring 15 located upstream thereof is not self-cleaned.